Plasticity of neurons is important for compensation in neuronal circuitry following lesions, neuronal attrition in aging, environmental influences in adulthood and developmental organization of connectivity. Although sprouting to restore the number of synaptic contacts is considered the common form of plasticity in the nervous system, changes in the size of synaptic sites now also appear to be an important mechanism for almost immediate response following Purkinje cell deafferentation. This study addresses the question of whether other neurons also have this potential for rapid alteration in the size of remaining synaptic sites as a product of a constancy in total postsynaptic contact area on each target neuron. The study is designed to test for this conservation principle by quantitative analysis of two other regions of the brain, namely the predominent neurons of the superior cervical ganglion and the granule cells of the hippocampus. Following reductions in the principal afferents to these two target neurons, the relationship between the number of afferent input sites on target neurons and the average size of individual postsynaptic membrane specializations will be quantitated. The values from a series of different reduction levels will be used to define whether the total area of contact on each neuron remains constant regardless of the amount of afferent contacts on each of the target neurons. The results will establish: (1) whether other neurons (besides Purkinje cells) may be able to re-organize connectivity through rapid redistribution of macromolecules for remaining receptor membrane sites, (2) whether there is support for a hypothesis that a mechanism of rapid plasticity is based on intrinsic control over target sites and (3) whether the post-synaptic sites in turn induce the afferents to produce more synaptic vesicles and a larger total presynaptic contact area.